1. Field of Invention
The present invention is directed to a method and apparatus for avoiding spurious signal receiver desensitizing. More particularly, the present invention is directed to a method and apparatus for reducing interference caused by spurious signals in a communication device.
2. Description of Related Art
Presently, electronic devices employ circuits operating at a rate set by a system clock. The system clock is set by a quartz crystal-based reference oscillator. To insure that the system clock is precise, the reference oscillator typically is tuned. The system clock is used by logic circuitry throughout the electronic device to ensure that they operate in synchronization.
Unfortunately, a problem exists with such devices such as a communication device in that the system clock generates unwanted spurious signals that interfere with the decoding of information from received signals by a receiver of the communication device. For example, a communication device operating in the GSM (Group Special Mobile) radiotelephone system employs a 13 MHz system clock. The 72nd and 73rd harmonics of the 13 MHz system clock are within the GSM downlink channels 5 and 70. When the communication device is tuned to channel 5 or channel 70, the harmonics of the system clock are radiated into the receiver of the communication device. The spurious energy causes the receiver to become desensitized to the information in the received signal resulting in substantial performance degradation in the form of high bit error rates.
Existing methods for reducing receiver desensitization caused by the harmonics of the system clock include shielding the transceiver of the communication device with additional material. For example, silver paint, conductive gaskets, beryllium copper fingers, and filters for filtering the clock signal can be employed in the communication device. Unfortunately, such materials add weight, size, and cost to the communication device and require time-consuming circuit board redesigns and manual manufacturing operations. Such manual manufacturing operations can include manual soldering of antenna assemblies, manual placement of gaskets, and desensitization testing.
Another method for reducing receiver desensitization in a communication device is disclosed in U.S. Pat. No. 5,745,848, issued on Apr. 28, 1998, which is herein incorporated by reference. This method generates the system clock from a phase lock loop circuit, and shifts the system clock signal from a first frequency to a second frequency when the communication device is to be tuned to a channel subject to receiver desensitization. Unfortunately, while this method may reduce receiver desensitization, there are drawbacks. In particular, timing errors occur when the reference phase locked loop circuit is programmed away from the nominal operating frequency. Thus, the method must compensate for these timing errors.
The ways in which timing error can impact performance of the communication system include the effect on time of arrival (TOA) measurement. One of the uses of TOA is for determining the location of the user relative to the nearby base stations, as with the extended observed time difference (E-OTD) measurements in GSM networks. A location accuracy requirement of 50 meters is anticipated, which for a single TOA measurement corresponds to timing accuracy of 166 nS. Within a total system timing error budget an allocated timing error due to clock frequency offsets may be on the order of 10 nS. Therefore, problems with timing error can significantly impact the performance of a communication system.
Thus, there is a need for a method and apparatus for improving the reduction of interference caused by spurious signals in a communication device.